1. Field of the Invention
This invention relates to a video signal reproduction apparatus for reproducing a video signal from a frequency multiplexed signal recorded on a video tape or other recording medium, and a time base correcting device for correcting an error in time base occurring in the video signal reproduction apparatus.
2. Description of the Related Art
FIG. 1 is a block diagram showing a conventional video signal reproduction apparatus. In FIG. 1, indicated at 1 is a magnetic head, at 2 a low-pass filter, at 3 a frequency converting circuit, at 5 a high-pass filter (hereinafter referred to as an HPF), at 6 an equalizing circuit, and at 7 a demodulating circuit. The magnetic head 1 is adapted for reading out, a frequency multiplexed signal S recorded on a magnetic video tape. The low-pass filter 2 is adapted for extracting a low-band converted carrier chrominance signal CL from the frequency multiplexed signal S outputted from the magnetic head 1. The frequency converting circuit 3 is adapted for converting the frequency of the low-band converted carrier chrominance signal CL so as to output an original high-band carrier chrominance signal C to a carrier chrominance signal output terminal 4. The HPF 5 is adapted for extracting a frequency-modulated luminance signal (hereinafter referred to as an FM luminance signal) YFM from the frequency multiplexed signal S outputted from the magnetic head 1. The equalizing circuit 6 is adapted for correcting a frequency-amplitude characteristic of the FM luminance signal YFM. The demodulating circuit 7 is adapted for demodulating the FM luminance signal YFM whose freuqnecy-amplitude characteristic is corrected, and feeding a reproduced luminance signal (hereinafter referred to merely as a luminance signal) Y to a luminance signal output terminal 8.
Next, there will be described an operation of the conventional video signal reproduction apparatus. The magnetic head 1 reads out the frequency multiplexed signal S recorded on the magnetic tape, in which signal the FM luminance signal YFM and the low-band converted carrier chrominance signal CL are multiplexed, and feeds the frequency multiplexed signal S to the low-pass filter 2 and the HPF 5. The low-pass filter 2 has a filter characteristic which attenuates a high frequency band in which the FH luminance signal YFM lies while passing a low frequency band in which the low-band converted carrier chrominance signal CL lies. Accordingly, the low-pass filter 2 extracts the low-band converted carrier chrominance signal CL from the frequency multiplexed signal S and feeds the same to the frequency converting circuit 3, The frequency converting circuit 3 converts the frequency of the low-band converted carrier chrominance signal CL so that the low-band converted carrier chrominance signal CL lies in a frequency band centering a chrominance sub-carrier, and outputs the original carrier chrominance signal C. On the other hand, the HPF 5 has a filter characteristic which attenuates the low frequency band in which the low-band converted carrier chrominance signal CL lies while passing the high frequency band in which the FM luminance signal YFM lies. Accordingly, the HPF 5 extracts the FM luminance signal YFM from the frequency multiplexed signal S and feeds the same to the equalizing circuit 6.
The frequency of the FM luminance signal YFM consists of innumerable wave components in an upper sideband (hereinafter referred to as upper wave components) and those in a lower sideband (hereinafter referred to as lower wave components) centering carrier waves (for example, 5.4 MHz to 7.0 MHz in the case of an S-VHS system). Here, it is assumed for the sake of simplicity that a frequency comprises a single carrier wave, and respectively one upper wave component and one lower wave component centering the single carrier wave as shown in FIG. 2. In the recording operation, both the upper wave component and the lower wave component are at the same level, which is lower than the level of the carrier wave. In a reproduction operation, there is obtained the FM luminance signal YFM having a frequency spectrum in which the level of the upper wave component is reduced under the influence of a characteristic of an electromagnetic conversion system as shown in FIG. 2(a). The equalizing circuit 6 corrects the FM luminance signal YFM having the frequency spectrum shown in FIG. 2(a) with an equalizer characteristic as shown in FIG. 2(b) to obtain an FM luminance signal YFM having a frequency spectrum shown in FIG. 2(c), and feeds the resultant FM luminance signal YFM to the demodulating circuit 7. The demodulating circuit 7 demodulates the corrected FM luminance signal YFM so as to reproduce the luminance signal Y, and outputs the reproduced luminance signal Y to the luminance signal output terminal 8.
Incidentally, when the video signal is reproduced directly from the frequency multiplexed signal S outputted from the magnetic head 1, the luminance signal Y includes the base variation. The time base variation can be removed by a servo circuit provided in a drive system for driving the magnetic tape. Alternatively, the time base variation can be removed by incorporating a time base correcting device into the video signal reproduction apparatus.
FIG. 3 is a block diagram showing a video signal reproduction apparatus provided with a conventional time base correcting device. In FIG. 3, indicated at 9 is a delay circuit, at 111 a variable delay circuit, at 112 a synchronizing signal separating circuit, at 113 an automatic frequency controller (hereinafter referred to as an AFC), at 114 a phase comparator circuit, at 115 a low pass filter (hereinafter referred to as an LPF), at 116 a voltage control oscillator (hereinafter referred to as a VCO), and at 119 a synchronizing signal separating circuit. The delay circuit 9 is adapted for delaying the carrier chrominance signal C by a period which is equal to a mean delay period in the variable delay circuit 111. The variable delay circuit 111 is adapted for delaying the luminance signal Y outputted from the demodulating circuit 7 by a period corresponding to an oscillating frequency of the VCO 116. The synchronizing signal separating circuit 112 is adapted for separating a horizontal synchronizing signal H1 from the delayed luminance signal Y1. The AFC 113 is adapted for generating a reference horizontal synchronizing signal HR having a fixed frequency from the horizontal synchronizing signal H1 outputted from the synchronizing signal separating circuit 112. The phase comparator circuit 114 is adapted for comparing the phase of the horizontal synchronizing signal H2 sent from the synchronizing signal separating circuit 119 and that of the reference horizontal synchronizing signal HR sent from the AFC 113, and outputting a time base error signal E. The LPF 115 is adapted for removing a noise component from the time base error signal E. The VCO 116 is adapted for oscillating at a frequency corresponding to an error amount indicated by the time base error signal E having its noise component removed therefrom. The synchronizing signal separating circuit 119 is adapted for separating the horizontal synchronizing signal H2 from the luminance signal Y outputted from the demodulating circuit 7. Other elements identical to those shown in FIG. 1 are indicated by the same reference numerals. Elements indicated by reference numerals in their hundreds constitute the time base correcting device.
Next, there will be described an operation of the video signal reproduction apparatus provided with the time base correcting device. An operation of reproducing the carrier chrominance signal C and the luminance signal Y from the frequency multiplexed signal S outputted from the magnetic head 1 is the same as the one described with reference to FIG. 1. In this case, the luminance signal Y outputted from the demodulating circuit 7 is fed to the variable delay circuit 111 and the synchronizing signal separating circuit 119 respectively.
The synchronizing signal separating circuit 112 extracts only the horizontal synchronizing signal H1 from the luminance signal Y1 outputted from the variable delay circuit 111 by removing a video signal portion therefrom, and feeds the extracted synchronizing signal H1 to the AFC 113. The AFC 113 controls the horizontal synchronizing signal H1 so as to have a substantially fixed frequency by removing the frequency variation of the horizontal synchronizing signal H1. More specifically, the AFC 113 controls the horizontal synchronizing signal H1 so as to obtain the reference horizontal synchronizing signal HR having a stable frequency close to a horizontal scanning frequency, and feeds the reference horizontal synchronizing signal HR to the phase comparator circuit 114. On the other hand, the synchronizing signal separating circuit 119 extracts only the horizontal synchronizing signal H2 from the luminance signal Y outputted from the demodulating circuit 7 by removing the video signal portion therefrom, and feeds the extracted horizontal synchronizing signal H2 to the phase comparator circuit 114. The phase comparator circuit 114 compares the phase of the horizontal synchronizing signal H2 outputted from the synchronizing signal separating circuit 119 with that of the reference horizontal synchronizing signal HR outputted from the AFC 113, detects a time base error included in the horizontal synchronizing signal H2, and feeds the time base error signal E to the LPF 115. The LPF 115 has a filter characteristic which removes the noise component in the high frequency band and passes the low frequency band where the time base error component lies. Accordingly, the LPF 115 feeds the time base error signal E having the noise component removed therefrom. The VCO 116 oscillates at a frequency according to the level of the time base error signal E, and its oscillating frequency serves to correct the tithe base error included in the luminance signal Y inputted to the variable delay circuit 111. The variable delay circuit 111, the phase comparator circuit 114, and the VCO 116 operate to extend the delay period of the variable delay circuit 111 when the phase of the horizontal synchronizing signal H2 is ahead of that of the reference horizontal synchronizing signal HR, and to shorten the delay period thereof when the phase of the horizontal synchronizing signal H2 is behind that of the reference horizontal synchronizing signal HR. The delay circuit 9 delays the carrier chrominance signal C by a period which is equal to a mean delay period in the variable delay circuit 111. It will be noted that the delay circuit 9 may be provided before the frequency converting circuit 3.
FIG. 4 is a block diagram showing a video signal reproduction apparatus provided with a time base correcting device of another prior art. In this prior art, a phase comparator circuit, 114 compares the phase of a horizontal synchronizing signal H outputted from a synchronizing signal separating circuit 112 and that of a reference horizontal synchronizing signal HR outputted from the AFC 113.
In this case, a time base error is corrected in a manner as described below. Firstly, a demodulating circuit 7 demodulates an FM luminance signal YFM to reproduce a luminance signal Y, and outputs the reproduced luminance signal. Y to a luminance signal output terminal 8. Simultaneously, the demodulating circuit 7 feeds the luminance signal Y to a synchronizing signal separating circuit 112. The synchronizing signal separating circuit. 112 extracts only the horizontal synchronizing signal H from the luminance signal Y by removing a video signal portion therefrom, and feeds the extracted horizontal synchronizing signal H to an AFC 113 and the phase comparator circuit. 114. The AFC 113 controls the horizontal synchronizing signal H so as to have substantially fixed frequency by removing frequency variation of the horizontal synchronizing signal H. That is to say, the AFC 113 controls the horizontal synchronizing signal H to obtain a reference horizontal synchronizing signal HR having a stable frequency close to a horizontal scanning frequency, and feeds the obtained reference horizontal synchronizing signal HR to the phase comparator circuit 114. The phase comparator circuit 114 compares the phase of the horizontal synchronizing signal H outputted from the synchronizing signal separating circuit 112 and that, of the reference horizontal synchronizing signal HR outputted from the AFC 113, detects a time base error included in the horizontal synchronizing signal H, and feeds a time base error signal E to an LPF 115. The LPF 115 has a filter characteristic which removes a noise component in the high frequency band and passes the low frequency band where the time base error component lies. Accordingly, the LPF 115 feeds the time base error signal E having a noise component removed therefrom to a VCO 116. The VCO 116 oscillates at a frequency according to the level of the time base error signal E, and its oscillating frequency serves to correct the time base error included it, the frequency multiplexed signal S inputted to a variable delay circuit 111. The variable delay circuit 111, the phase comparator circuit 114, and the VCO 116 operate to extend the delay period of the variable delay circuit 111 when the phase of the horizontal synchronizing signal H is ahead of that of the reference horizontal synchronizing signal HR, and to shorten the delay period thereof when the phase of the horizontal synchronizing signal H is behind that of the reference horizontal synchronizing signal HR. In this way, the time base variation included in the frequency multiplexed signal S outputted from the magnetic head 1 is removed. As a result, the time base error of the luminance signal Y can be corrected.
FIG. 5 is a block diagram showing a video signal reproduction apparatus provided with a time base correcting device of still another prior art. In this prior art, a variable delay circuit 111 is provided between an equalizing circuit 6 and a demodulating circuit 7. A synchronizing signal separating circuit 119 separates a horizontal synchronizing signal H2 from a luminance signal Y2 outputted from a demodulating circuit 123 provided after the equalizing circuit 6.
In this case, a time base error is corrected in a manner as described below. Firstly, a demodulating circuit 7 demodulates an FM luminance signal YFM to reproduced a luminance signal Y, and outputs the reproduced luminance signal Y to a luminance signal output terminal 8. Simultaneously, the demodulating circuit 7 feeds the luminance signal Y to a synchronizing signal separating circuit 112. The synchronizing signal separating circuit 112 extracts only the horizontal synchronizing signal H from the luminance signal Y by removing the video signal portion therefrom, and feeds the extracted horizontal synchronizing signal H to an AFC 113. The AFC 113 controls the horizontal synchronizing signal H so as to have substantially fixed frequency by removing frequency variation of the horizontal synchronizing signal H. That is to say, the AFC 113 controls the horizontal synchronizing signal H to obtain a reference horizontal synchronizing signal HR having a stable frequency close to a horizontal scanning frequency, and feeds the obtained reference horizontal synchronizing signal HR to the phase comparator circuit 114. On the other hand, the demodulating circuit 123 demodulates the FM luminance signal YFM outputted from the equalizing circuit 6, and feeds the luminance signal Y2 to the synchronizing signal separating circuit 119. The synchronizing signal separating circuit 119 extracts only the horizontal synchronizing signal H2 from the luminance signal Y2 by removing the signal portion therefrom, and feeds the extracted horizontal synchronizing signal H2 to the phase comparator circuit 114. The phase comparator circuit 114 compares the phase of horizontal synchronizing signal H2 outputted from the synchronizing signal separating circuit 119 and that of reference horizontal synchronizing signal HR outputted the AFC 113, detects a time base error included in the horizontal synchronizing signal H2, and feeds a time base error signal E to an LPF 1115. The LPF 115 has a filter characteristic which removes the noise component in the frequency band and passes the low frequency band where the time base error component, lies. Accordingly, the LPF 115 feeds the time base error signal E having the noise component removed therefrom to a VCO 116. The VCO 116 oscillates at a frequency according to the level of the time base error signal E, and its oscillating frequency serves to compensate for the time base variation included in the luminance signal YFM inputted to the variable delay circuit 111. In this way, the time base variation of the FM luminance signal YFM is removed. As a result, the time base error of the luminance signal Y can be coerce, ted. The delay circuit 9 delays the carrier chrominance signal C by a period which is equal to a mean delay period in the variable delay circuit 111. It will be noted that the delay circuit 9 may be provided before the frequency converting circuit 3.
FIG. 6 is a block diagram showing a video signal reproduction apparatus provided with a time base correcting device of further another prior art, In FIG. 6, indicated at 121 is a HPF for extracting an FM luminance signal YFM from a frequency multiplexed signal S output, ted from a magnetic head 1, at 122 a equalizing circuit for correcting a frequency-amplitude characteristic of the FM luminance signal YFM outputted from the HPF 121, and at 123 a demodulating circuit for demodulating the corrected FM luminance signal YFM to reproduce a luminance signal Y2 and outputting the reproduced luminance signal Y2 to a synchronizing signal separating circuit 119. Other elements identical to those shown in FIG. 5 are indicated by the same reference numerals. However, in this case, a variable delay circuit 111 is provided between the magnetic head 1 and, an LPF 2 and an HPF 5. A synchronizing signal separating circuit 112 is provided after a demodulating circuit 7, and the synchronizing signal circuit 119 is provided after the second demodulating circuit 123.
Next, there will be described an operation of the above video signal reproduction apparatus. The magnetic head 1 reads out a frequency multiplexed signal S recorded on a magnetic tape, and feeds the read out signal S to the variable delay circuit 111. The variable delay circuit 111 corrects a time base error of the frequency multiplexed signal S as described below, and feeds the corrected frequency multiplexed signal S to the LPF 2 and the HPF 5. An operating of reproducing a carrier chrominance signal C and a luminance signal Y from the frequency multiplexed signal S is similar to the operation of reproducing those signals from the frequency multiplexed signal S outputted from the magnetic head 1 described with reference to FIG. 1.
In this case, a time base error is corrected in a manner as described below. Firstly, the demodulating circuit 7 demodulates the FM luminance signal YFM to reproduce a luminance signal Y, and outputs the reproduced luminance signal Y to a luminance signal output terminal 8. Simultaneously, the demodulating circuit 7 feeds the luminance signal Y to a synchronizing signal separating circuit 112. The synchronizing signal separating circuit 112 extracts only the horizontal synchronizing signal H from the luminance signal Y by removing the video signal portion therefrom, and feeds the extracted horizontal synchronizing signal H to an AFC 113. The AFC 113 controls the horizontal synchronizing signal H so as to have substantially fixed frequency by removing frequency variation of the horizontal synchronizing signal H. That is to say, the AFC 113 controls the horizontal synchronizing signal H to obtain a reference horizontal synchronizing signal HR having a stable frequency close to a horizontal scanning frequency, and feeds the obtained reference horizontal synchronizing signal HR to the phase comparator circuit 114. The HPF 121 has a filter characteristic which attenuates the low frequency band where the low band converted carrier chrominance signal CL lies and passes the high frequency band where the FM luminance signal YFM lies. The HPF 121 extracts the FM luminance signal YFM from the frequency multiplexed single S outputted from the magnetic head 1, and feeds the extracted FM luminance signal YFM to the equalizing circuit 122. The equalizing circuit 122 corrects the FM luminance signal YFM having a frequency spectrum shown in FIG. 2(a) with an equalizer characteristic shown in FIG. 2(b), and feeds the FM luminance signal YFM having a frequency spectrum shown in FIG. 2(c) to the demodulating circuit 123. The demodulating circuit 123 demodulates the FM luminance signal YFM fed thereto to reproduce a luminance signal Y2, and feeds the reproduced luminance signal Y2 to the synchronizing signal separating circuit 119. The synchronizing signal separating circuit 119 extracts only the horizontal synchronizing signal H2 from the luminance signal Y2 by removing a video signal portion therefrom, and feeds the extracted horizontal synchronizing signal H2 to the phase comparator circuit 114. The phase comparator circuit 114 compares the phase of the horizontal synchronizing signal H2 fed from the synchronizing signal separating circuit 119 and that of the reference horizontal synchronizing signal HR fed from the AFC 113, and feeds the time base error signal E to the LPF 115. The LPF 115 has a filter characteristic which removes the noise component in the high frequency band and passes the low frequency band where the time base error component lies. Accordingly, the LPF 115 feeds the time base error signal E having the noise component removed therefrom to the VCO 116. The VCO 116 oscillates at a frequency according to the level of the time base error signal E, and its oscillating frequency serves to correct the time base error included in the frequency multiplexed signal S inputted to the variable delay circuit 111. The variable delay circuit 111, the phase comparator circuit. 114, and the VCO 116 operate to extend the delay period of the variable delay circuit 111 when the phase of the horizontal synchronizing signal H is ahead of that of the reference horizontal synchronizing signal HR, and to shorten the delay period thereof when the phase of the horizontal synchronizing signal H is behind that of the reference horizontal synchronizing signal HR. In this way, the time base variation included in the frequency multiplexed signal S outputted from the magnetic head 1 can be removed. As a result, a time base error of the luminance signal Y can be corrected.
Since the conventional video signal reproduction apparatus is constructed as described above, the equalizing circuit 6 provided before the demodulating circuit 7 for demodulating the FM luminance signal YFM operates to raise the level of the upper wave components of the FM luminance signal YFM. Accordingly, a signal-to-noise ratio (S/N) of a horizontal synchronizing signal portion of the luminance signal Y becomes degraded particularly fin a long-time recording mode. This has caused the following problems: The frequency converting circuit 3 in a chrominance signal processing system using a horizontal synchronizing signal of the luminance signal Y operates in an unstable manner. A synchronous processing system operates in an unstable manner in an image receiver or the like to which an output signal of this apparatus is inputted.
Further, since the video signal reproduction apparatus provided with a conventional time base correcting device is constructed as described above, an S/N of the horizontal synchronizing signal portion of the luminance signal Y becomes degraded. Accordingly, it, has been a problem that the degree of the time base error is rather increased on the ground that accurate time base error information cannot be extracted, or wrong time base error information is extracted in the phase comparator circuit 114 of the time base correcting device.